Aspects of the cell biology and immunology of host-parasite interactions among several intra- and extracellular parasitic protozoa are being investigated. Leishmania sp. and Trypanosoma sp. are being used as models of intracellular and extracellular parasitism, respectively. Emphasis is placed upon: 1) determining some of the basic chemical and antigenic properties of parasite surface membranes; 2) ascertaining the nature and extent of the interactions of parasite surfaces with specific host cell types; 3) defining the basic mechanisms involved in the intracellular survival and multiplication of parasites within host cells, and 4) attempts to determine the means by which parasites circumvent host immuno-defense systems. Techniques used in these studies include: cell fractionation, ultracyto- and immunochemistry, electron microscopy, serology, lectin binding, gel immunoassays, and cell culture. BIBLIOGRAPHIC REFERENCES: Chang, K.-P. and Dwyer, D.M.: Multiplication of a human parasite (Leishmania donovani) in phagolysosomes of hamster macrophages in vitro. Science 193: 678-680, 1976. Dwyer, D.M.: Leishmania donovani: surface membrane carbohydrates of promastigotes. Exp. Parasitol. 41: 341-358, 1977.